The Iranian government has arrested relatives of Persian-language journalists working abroad for the BBC in a bid to silence them, the British broadcaster said on Friday.
BBC director general Mark Thompson said the sister of a BBC Persian journalist was arrested last week and held in solitary confinement on unspecified charges at Evin Prison in Tehran, before being released on bail.
"Her treatment was utterly deplorable and we condemn it in the strongest terms," Thompson wrote in a blog, adding that it was only the latest incident "in a campaign of bullying and harassment by the Iranian authorities".
Human Rights Watch also raised concerns about the arrest, with its Middle East director Sarah Leah Whitson warning it was "part of a wider campaign to harass Iranian journalists by putting pressure on them and their families".
She said Tehran was sending a clear message "that the government's long arm of repression can extend well beyond borders."
It is not the first time the BBC has complained about Iranian harassment of its journalists but Thompson said the past few months had seen "increased levels of intimidation alongside disturbing new tactics".
"In recent months a number of relatives of members of BBC Persian staff have been detained for short periods of time by the Iranian authorities and urged to get their relatives in London to either stop working for the BBC, or to 'cooperate' with Iranian intelligence officials," he said.
Relatives' passports had been confiscated, preventing them leaving Iran, while BBC staff had been accused in the Iranian media of offences such as sexual assault, drug trafficking and converting from Islam to Christianity.
Thompson called on Tehran to repudiate the actions of its officials and urged governments and international bodies to help stop "this campaign of intimidation, persistent censorship and a disturbing abuse of power".
"It is behaviour that all people who believe in free and independent media should be deeply concerned about," the BBC chief said.
BBC Persian, based in London, is a multimedia news and information service for Persian speakers in Iran, Afghanistan, Tajikistan and around the world.
Iran has frequently accused the BBC of fuelling the unrest that broke out following the disputed re-election of President Mahmoud Ahmadinejad in 2009.